Antenna reflector system



Dec. 27, T H C RK ANTENNA REFLECTOR SYSTEM Filed Oct. 12, 1945 INVEN TOR. 'Z VOZv;(%f/ 4,, a,

ATTORNEY Patented Dec. 27, 1949 ANTENNA REFLECTOR SYSTEM Trevor H. Clark, Boonton, N.

eral Telephone and Radio J assignor to Fed Corporation, New

York, N. Y., a corporation of Delaware Application October 12, 1945, Serial No. 622,045

2 Claims.

This invention relates to antenna reflectors and more particularly to a method and means for providing an increase in. the apparent electrical size of the reflector without having to resort to a physical extension thereof.

' The use of reflectors with antennasis based on the knowledge that if a sheet of metal is interposed in an electromagnetic field, currents will flow. The magnitude of the currents will depend upon the characteristics of the conductor such as conductivity, skin efiect, dimensions of the conductor, wavelength of the radiation and position of the reflector or metallic object with respect to the vector quantities of the electromagnetic field.

In the past, when directive radiation or radiation sensitivity was required, the combined use of an antenna and a suitable reflector was indicated. While the result of such a combination at comparatively high frequencies will not be unduly ineflicient reflectors designed for handling a broad band including low frequencies showed resonance effects. This is due to the fact that some parts of the reflector at such low frequencies approach the dimensions of half wavelengths. such resonance effects are hard to avoid unless the reflector is made unduly large as the case will be for the lower frequencies. This type of resonance in a reflector gives rise to currents in the reflector which produce back radiation and cause a distortion of the antenna field and undesirable back radiation. This reradiation may be explained, in one way by considering that current flows around the edges of the reflector from the front side to the other side, thereof.

It is an object of the invention to provide a reflector for use with an antenna to which has been given an apparent increase in electrical dimensions without a change in the physical dimensions, thereof.

It is another object to provide a reflector of the above-defined type which avoids the production of resonant reradiation productive currents therein.

Still another object is to reduce the dimensions of the reflector for a given wavelength.

A still further object is to provide an antenna reflector system which is less frequency dependent than those used heretofore.

In accordance with the invention, I provide for an increase in the apparent size of a reflector for use with an antenna by means of a comparatively high resistance reflecting surface which either forms the whole or a part of the 2 reflector. In accordance with certain other features of the invention, portions of the reflecting surface may be made of good conducting and partly of highly resistive material to provide reflecting properties in accordance with specific requirements.

These and other features and objects of my invention will become more apparent upon consideration of the following detailed description of an embodiment to be read in connection with the accompanyin drawings in which:

Fig. 1 is a plan view of a reflector and associated antenna in accordance with my invention;

Fig. 2 is another form of reflector for use with an antenna; and

Fig. 3 is still a further form of a reflector which may be used with an antenna, in accordance with my invention.

Referring to the drawings, the reflector of Fig. 1 comprises a basic resistive reflecting surface I which may consist of a flat portion of nonconducting material such as wood or plastic which has been sprayed with a collodial graphite or other conductive paint or of a layer of oxidized steel or other material which is resistive to high frequency currents. Another portion of the reflector of Fig. 1 arranged at the center of the base I, is formed of a highly conductive material such as copper or aluminum sheet 2, a dipole antenna 3 preferably designed for broad band operation being suitably disposed with respect to the center of the reflector as indicated.

In Fig. 2, the reflector consists of a surface 4 which is wholly resistive as defined above, While the reflector of Fig. 3 comprises a resistive base surface 5 and numerous conductive portions 6 disposed on or in the base 5.

In all three instances the apparent size of the reflectors has been increased by the use of a highly resistive reflecting surface. The high resistance of such a surface tends to attenuate induced currents and prevent localized currents which might lead to undesired effects such as polarization errors, back radiation and other effects. Where it is desired to obtain certain reflection properties for the reflector certain portions thereof may be made of good conductive material the rest being resistive as suggested in Figs. 1 and 3. It will be understood, of course, that other configurations and proportions of the conductive and resistive portions of a reflector than those shown are possible. In addition, the shape of both the conducting as well as of the resistive portions may be formed with rounded, squared, oval or serrated contours as desired.

The introduction of resistive portions in reflectors of the above type will efiectively decouple the two sides of the reflector sheet by reducing the flow of currents around the edges from one to the other side. This in turn avoids the formation of distorted fields which otherwise would take place at the comparatively low frequenciesiwhi'cli usually cause a;.=.pick-up from/the back of'the reflecting screen. Thus, the distortion of the antenna field which is commonly present due to the presence of the reflecting screen itself is greatly minimized. The: resistive? reflector therefore, is useful for wide band applications allowing for smaller dimensions of the reflector for the same frequency as. compared;

with a reflector consisting of a highly conductive surface alone.

It will be seen from the above thatv by, cone structing the reflector with a high resistance sur-- face its effective electrical size is increased for better-accommodation of lower-'- frequencies without having to resort to physical increase of the size:

' While I- have described above the principles of my invention inconnection with specific apparatus it is to be clearly understood that this description i's-made only'by way of example and not as-a limitation: onthe' scope of my invention as defined inthe objects and the accompanying" claims;

I claim:

1. A directive wide band antenna system comprising a wide band antenna, a reflector of predetermined shape for increasing the directivity of the radiation from said antenna mounted behind said antenna, said reflector having a cen- .tral. reflecting portion of. highly conductive material andianaouter reflecting portion surrounding said central portion and of highly resistive material for increasing the apparent size of said reflector.

2-1 An antenna according to claim 1 in which said. central reflecting portion is of copper and said outer. reflecting portion of plastic.

- TREVOR H. CLARK.

REFERENCES CITED The following references are of record in the file of this patent:

Lindenbladl Apr.v 28, 1942 

